Noise power estimation apparatus, noise power estimation method and signal detection apparatus

ABSTRACT

A noise power estimation apparatus is disclosed. The noise power estimation apparatus includes: a part for calculating correlation between a received signal and a pilot signal so as to obtain a received power of the pilot signal for each path; a part for removing a multipath interference component from the received power of the pilot signal by using a predetermined power ratio between the pilot signal and a data signal so as to obtain a corrected received power of the pilot signal; a part for estimating an estimated total power of the pilot signal and the data signal included in the received signal based on the corrected received power and the predetermined power ratio; and a part for subtracting the estimated total power from a total power of the received signal so as to obtain a noise power.

CROSS-REFERENCES TO RELATED APPLICATION

This application is a divisional of and is based upon and claims thebenefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 11/128,188,filed May 13, 2005, and claims the benefit of priority under 35 U.S.C. §119 from Japanese Patent Application No. 2004-144181, filed May 13,2004, the entire contents of each which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a field of radio communications. Moreparticularly, the present invention relates to a signal detectionapparatus used in a radio receiver, and an apparatus and a method forestimating a noise power used for signal detection.

2. Description of the Related Art

In the field of the radio communications, research and development arebeing conducted for realizing large-capacity high-speed informationcommunications of current and the next generation or later. Especially,the Multi Input Multi Output (MIMO) scheme for increasing thecommunication capacity is receiving attention.

FIG. 1 is a schematic diagram of a communication system of the MIMOscheme including a transmitter 102 and a receiver 104. In the MIMOscheme, different signals are transmitted from plural transmissionantennas 106-1˜N at the same time with the same frequency. Thesetransmission signals are received by plural receiving antennas 108-1˜N.For the sake of simplicity, each number of the transmission antennas andthe receiving antennas is N, but the numbers may be different betweenthe transmitter and the receiver.

FIG. 2 shows a part relating to signal separation in the receiver 104.Roughly speaking, the receiver receives signals transmitted from theplural transmission antennas with the plural receiving antennas, asignal detection part detects the transmission signals, and thetransmission signals are separated to signals for each transmissionantenna. The signal separation is performed by signal processing in thetwo-dimensional frequency domain by using the Minimum Mean Square Error(MMSE) method. Received signals (r) received by each receiving antennaare supplied to a channel estimation part 202. The channel estimationpart 202 obtains channel impulse responses or channel estimation valuesbetween the transmission antennas and the receiving antennas. Thechannel estimation result is supplied to a fast Fourier transform part(FFT) 204 to be converted to information in the frequency domain andsupplied to a weight generation part 206. A weight W generated in theweight generation part 206 is represented by a following equation, forexample:

W=(HH ^(H)+σ² I)⁻¹ H   (1)

wherein, “H” indicates a channel matrix having channel impulse responsesas its matrix elements, “I” indicates a unit matrix, and a σ² indicatesa noise power arising in the receiver. The superscript “H” indicatestransposed conjugate.

The received signals (r) are also supplied to a fast Fourier transformpart 210, and are converted to signals in the frequency domain so thatthe signals are supplied to a MMSE equalizing part 208. The MMSEequalizing part 208 substantially performs signal separation bymultiplying the received signals in the frequency domain by a weightW^(H). The separated signals are supplied to an inverse fast Fouriertransform part 212 so that the signals are converted to signals in thetime domain, and the signals are output as estimated signals t that areseparated for each transmission antenna.

Japanese Laid-Open Patent Application No. 2003-124907 discloses using asignal-to-noise ratio in the MIMO scheme.

For correctly estimating the transmission signals, it is necessary toperform signal separation with very high precision in the signaldetection part. For this purpose, it is necessary to correctly obtainthe weight W. As shown in the equation (1), since the weight W islargely affected by the channel matrix, the channel estimation needs tobe performed correctly in the channel estimation part 202. In addition,according to the equation (1), the weight W is affected by the noisepower σ, the noise power needs to be obtained correctly. However,according to the conventional technology of this field, little attempthad been made to obtain the noise power correctly. However, in futureproducts for high-capacity and high-speed information transmission,there is a risk in that signal separation is not properly performed dueto lack of estimation accuracy of the noise power.

SUMMARY OF THE INVENTION

The present invention is contrived to solve at least one of theabove-mentioned problems. An object of the present invention is toprovide a noise power estimation apparatus, a noise power estimationmethod and a signal detection apparatus for estimating a chip noisepower used for weight calculation in the MMSE equalizer with highprecision.

The object is achieved by a noise power estimation apparatus, including:

-   -   a part for calculating correlation between a received signal and        a pilot signal so as to obtain a received power of the pilot        signal for each path;    -   a part for removing a multipath interference component from the        received power of the pilot signal by using a predetermined        power ratio between the pilot signal and a data signal so as to        obtain a corrected received power of the pilot signal;    -   a part for estimating an estimated total power of the pilot        signal and the data signal included in the received signal based        on the corrected received power and the predetermined power        ratio; and    -   a part for subtracting the estimated total power from a total        power of the received signal so as to obtain a noise power.

According to the present invention, the noise power used for weightcalculation in the MMSE equalizer and the like can be estimated withhigh precision.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a communication system of the MIMOscheme;

FIG. 2 shows a conventional MMSE equalization apparatus of thetwo-dimensional frequency domain;

FIG. 3 shows a MMSE equalization apparatus of the two-dimensionalfrequency domain according to an embodiment of the present invention;

FIG. 4 is a block diagram of a noise estimation part according to anembodiment of the present invention;

FIG. 5 is a conceptual diagram for explaining relationships amongtransmission signals, received signals and the multipath interferencecomponent;

FIG. 6 is a diagram showing power ratio between the pilot signal and thedata signal;

FIG. 7 is a diagram showing characteristics of impulse response of theroll-off filter;

FIG. 8 is a block diagram of the noise estimation part according to anembodiment of the present invention;

FIG. 9 shows a conceptual diagram of a multistage signal detectionapparatus according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention are describedwith reference to figures.

(Outline of the Embodiments)

According to an embodiment of the present invention, the noise power isestimated such that the effect of the multipath interference is removed.Thus, the noise power can be estimated more correctly compared with theconventional technology. Therefore, the weight used for signalseparation can be obtained correctly, so that the accuracy of the signalseparation can be improved.

According to an embodiment of the present invention, the noise power canbe recursively updated by using a recurrence formula including anoblivion coefficient. Thus, the noise power can be adaptively updatedaccording to a communication environment, so that the calculationaccuracy of the weights and the signal separation accuracy can befurther improved.

According to an embodiment of the present invention, the multipathinterference component can be obtained by accumulating, for plural pathsand for plural transmission antennas, a product of the received power ofthe pilot signal and a constant including the predetermined power ratio.Thus, the multipath interference component can be obtained easily andwith reliability.

First Embodiment

FIG. 3 shows a part relating to a signal detection apparatus in thereceiver 104. In outline, the receiver receives signals transmitted fromN transmission antennas with N receiving antennas, detects transmissionsignals, and separates signals for each transmission antenna. The signalseparation is performed by signal processing in the two-dimensionalfrequency domain by using the MMSE (Minimum Mean Square Error) method.Instead of using the frequency domain, equalization by MMSE can be alsoperformed in the time domain. But, in view of simplicity of calculation,it is desirable to perform signal processing in the frequency domainlike the present embodiment. Although both of the numbers of thetransmission antennas and the receiving antennas are N in thisembodiment, the numbers may be different.

The receiver includes a channel estimation part 302, a noise estimationpart 304, fast Fourier transform parts (FFT) 306 and 308, a weightgeneration part 310, a MMSE equalizing part 312 and an inverse fastFourier transform part (IFFT) 314.

The channel estimation part 302 receives received signals r=(r₁, . . . ,r_(N)) that are received by each corresponding receiving antenna. Thechannel estimation part 302 calculates channel impulse responses (CIR)or channel estimated values between the transmission antennas and thereceiving antennas based on the received signals and pilot signals.

The noise estimation part 304 estimates a noise power or a chip noisepower σ² based on the received signals. More detailed configurations andoperations of the noise estimation part 304 are described later.

Each of the fast Fourier transform parts 306 and 308 performs fastFourier transform on input signals so as to transform the signals tosignals in the frequency domain. On the other hand, the inverse fastFourier transform part 314 performs inverse fast Fourier transform oninput signals so as to transform the input signals to the signals in thetime domain.

The weight generation part 310 obtains the weight W used in the MMSEequalizing part 312 based on the channel estimation result and the noisepower. The weight W is represented by a following equation;

W=(HH ^(H)+σ² I)⁻¹ H   (2)

wherein “H” indicates a channel matrix having the channel impulseresponses as matrix elements, superscript “H” indicates a transposedconjugate, “I” indicates a unit matrix, and σ² indicates a noise poweroccurring in the receiver. The noise power ideally includes only a noisearisen in the receiver and does not include a noise applied in theoutside of the receiver (for example, signal interference applied in apropagation route). However, the noise power includes the noise in theoutside of the receiver in actuality. Therefore, the noise power needsto be precisely estimated in the following way. In the presentembodiment, since the number of the antennas is N for both of thereceiving side and the transmission side, each of the channel matrix Hand the weight matrix becomes a N×N square matrix. When the channelmatrix is a M×N matrix, HH^(H) becomes a M×M square matrix, and theweight matrix W becomes a M×N matrix. In this case, N represents thenumber of the transmission antennas, and M indicates the number ofreceiving antennas.

The MMSE equalizing part 312 multiplies the converted frequency domainsignals by the weight W^(H) so as to perform signal separation(t_(f)=W^(H)r_(f)), wherein r_(f) indicates signals obtained byconverting the received signals r to the frequency domain, and t_(f)indicates separated signals in the frequency domain. The separatedsignals are supplied to the inverse fast Fourier transform part 314 sothat the signals are transformed to signals in the time domain to beoutput as estimated transmission signals t=(t₁, . . . , t_(N)) that areseparated for each transmission antenna.

FIG. 4 is a block diagram of the noise estimation part 304 according toan embodiment of the present invention. The noise estimation part 304includes a total received signal power measuring part 402, a pilotreceived power estimation part 404, a multipath interference generationpart 406, a multipath interference removing part 408, a total receivedsignal power estimation part 410, a subtraction part 412 and anaveraging part 414.

The total received signal power measuring part 402 measures a totalreceived power R_(m) of signals received by a receiving antenna r_(m) asshown in the following equation:

R _(m) =E(|r _(m)(t)|²)

wherein E(•) indicates a process for calculating an average or anexpected value of an amount in the parentheses, and m is a parameter forspecifying a receiving antenna (1≦m≦M). In this embodiment, the number Mof the receiving antennas is the same as the number N of thetransmission antennas. The total received signal power measuring part402 obtains a total received power for each receiving antenna.

The pilot received power estimation part 404 calculates a received powerP_(nml) of a pilot signal for each path by the following equation:

$p_{n,m,l} = {{\frac{1}{N_{c}}{\sum\limits_{t = \tau_{l}}^{N_{c} - 1 + \tau_{l}}\; {{r_{m}(t)} \cdot {c_{n}^{*}\left( {t - \tau_{l}} \right)}}}}}^{2}$

wherein n is a parameter indicating a transmission antenna, l is aparameter that specifies a path in assumed L paths, τ_(l) indicates adelay amount of a l-th path, * indicates complex conjugate, Nc indicatesthe number of chips in one frame and specifies the number of chips or asize of a window of a range in which correlation calculation isperformed, c_(n)(t) is a code series indicating a pilot signal relatingto a n-th transmission antenna.

The multipath interference generation part 406 calculates multipathinterference components included in pilot signals for each path. FIG. 5is a conceptual diagram for explaining relationships among transmissionsignal, received signal and the multipath interference component. Forthe sake of simplicity, it is assumed that two transmission antennas Tx1and Tx2 transmit pilot signals c₁ and c₂ respectively, and the signalsare transmitted under a multipath propagation environment and receivedby a receiving antenna Rx1. In addition, it is assumed that there aretwo paths of a path 1 and a path 2. Although the number of antennas andthe number of paths are assumed as mentioned above, greater numbers canbe assumed. In this case, a power obtained based on a correlationbetween signals received by the receiving antenna Rx1 and the pilotsignal c₁ includes a multipath interference component of Tx1 from thepath 2 and a multipath interference component of Tx2 from the path 2 inaddition to a power of Tx1 from the path 1. When there are more paths,multipath interference components appear according to the number ofpaths. When the number of transmission antennas increase, multipathinterference components appear according to the increased number oftransmission antennas.

Not only the pilot signal but also a data signal transmitted with thepilot signal contribute to the multipath interference. When a signal istransmitted from a transmission antenna, a power ratio between the pilotsignal and the data signal is predetermined. For example, as shown inFIG. 6, a power ratio of the data signal to the pilot signal is α.Therefore, when the power for the pilot signal is determined, the powerof the data signal can be determined. According to the above-mentionedconsiderations, the multipath interference component generation part 406in FIG. 4 obtains the multipath interference components for each path.

The multipath interference removing part 408 calculates a correctedreceived power P′_(nml) of the pilot signal for each path by subtractingmultipath interference component from the received power of the pilotsignal for each path obtained in the pilot received power estimationpart 404 based on the following equation:

$p_{n,m,l}^{\prime} = {p_{n,m,l} - \frac{\sum\limits_{n^{\prime}}^{N}\; {\sum\limits_{l^{\prime},{l^{\prime} \neq l}}^{L}\; {\left( {1 + \alpha} \right)P_{n^{\prime},m,l^{\prime}}}}}{N_{c}}}$

wherein (1+α)P_(n′ml′) indicates a total power (of the pilot signal andthe data signal) of a l′-th path in signals from a n′-th transmissionantenna. As shown in this equation, summation relating to l′ isperformed for all paths excluding the own (l-th) path, summationrelating to n′ is performed for all transmission antennas, and, Ncindicates the number of chips in one frame and 1/Nc is introduced in aterm indicating the multipath interference component for obtainingmultipath interference per chip.

The total received signal power estimation part 410 estimates the totalreceived power of the pilot signals received by a m-th receiving antennaby further correcting the corrected received power of the pilot signalfor each path. The total received power of the pilot signals received bya m-th receiving antenna can be mostly calculated by adding, for allpaths and for all transmission antennas, the corrected received powerP′_(nml) of the pilot signal for each path. However, from the viewpointof improving the precision, it is desirable to further performcorrection. Generally, the signal received by each receiving antennaincludes a side lobe component in addition to a main lobe since thesignal passes through a roll-off filter (band limitation filter). Thus,the received power of the pilot signal includes the side lobe component,so that the amount of the received signal power is evaluated to belarger than an actual amount to some extent. Since the impulse responsecharacteristics of the roll-off filter are known, the side lobecomponent can be compensated based on the known responsecharacteristics. The impulse response characteristics h_(RC)(t) of theroll-off filter are shown as FIG. 7, for example, and is represented asthe following equation:

${h_{RC}(t)} = {\left( \frac{\sin \left( {\pi \; {t/{Tc}}} \right)}{\pi \; {t/{Tc}}} \right)\left( \frac{\cos \left( {\pi \; \alpha \; {t/{Tc}}} \right)}{1 - \left( {4\; \alpha \; {t/\left( {2\; {Tc}} \right)}} \right)^{2}} \right)}$

wherein α is a roll-off factor and α=0.22 in the example of FIG. 7, Tcindicates a chip period. Generally, the range (|t|≦Tc) of the one chipperiod can be associated with the main lobe (actual signal component ofthe path), and a range (|t|>Tc) other than the one chip period can beassociated with the side lobe.

The total received signal power estimation part 410 further corrects thecorrected received power P′_(nml) of the pilot signal for each pathaccording to the following equation, so as to estimate a total receivedpower P_(all,m) of the pilot signals received by the m-th receivingantenna:

$p_{{all},m} = {\frac{1}{N_{os}}{\sum\limits_{t = 0}^{N_{os} - 1}\; {\sum\limits_{n = 1}^{N}\; {\sum\limits_{c = {{- W}/2}}^{W/2}\; \begin{pmatrix}{\sum\limits_{l = 1}^{L}\; {\sqrt{\left( {1 + \alpha} \right)P_{n,m,l}^{\prime}} \cdot ^{j\; {\theta {({n,m,l})}}} \cdot}} \\{h_{Rc}\left( {t + \tau_{l} - \tau_{1} + {cN}_{os}} \right)}\end{pmatrix}^{2}}}}}$

wherein N_(os) indicates the number of oversampling and N_(os)=4 in thepresent example, θ(n,m,l) indicates a phase rotation amount of a l-thpath between n-th transmission antenna and a m-th receiving antenna(which does not contribute to power). Correction on the side lobecomponent mainly relates to summation for the parameters t and c. Byperforming summation for all paths (parameter l) and all antennas(parameter n), the total received power P_(all,m) of the pilot signalsreceived by the m-th receiving antenna can be estimated.

The subtraction part 412 calculates a noise power (chip noise power)σ_(m) ² of signals received by the m-th receiving antenna by subtractingthe estimated total received power P_(all,m) from the total receivedpower R_(m) of the pilot signals received by the m-th antenna as shownin the following equation:

σm2=Rm−Pall,m

wherein this process is performed for each receiving antenna.

The averaging part 414 averages the noise powers σm2 (m=1˜N), that areobtained for each received antenna, for all receiving antennas so as toobtain a noise power σ2 of the receiver. Since the effect of themultipath interference is removed in the obtained noise power, the noisepower is estimated more correctly compared with the conventionaltechnology. Therefore, the weight generation part 310 of FIG. 3 cancalculate the weight properly. In addition, from the viewpoint offurther improving the precision, the averaging part 414 may update thenoise power recursively by using an oblivion coefficient a. That is, thenoise power can be updated according to σk+12=a·k2+(1−a)σk−12. Theupdating method of the noise power is not limited to the above mentionedmethod using the oblivion coefficient. The update can be performed byusing other recurrence formulas. Further, weighting coefficient can beappropriately adjusted when averaging the noise powers in the receivingantennas.

Second Embodiment

In the first embodiment, the noise power σ2 is estimated based onreceived signals on which despreading has not been performed. On theother hand, in the second embodiment described in the following, thenoise power σ2 is estimated based on despread received signals.

FIG. 8 is a block diagram of the noise estimation part according to thepresent embodiment. The noise estimation part includes M noise powerestimation parts 802-1˜M each being provided for a correspondingreceiving antenna, and an averaging part 804 among antennas. For thesake of simplicity, FIG. 8 shows only two noise power estimation parts802-1 and 802-m. Since each of the noise power estimation parts has thesame configuration and operates in the same way, the noise powerestimation part 802-1 is described as an example.

The noise power estimation part 802-1 includes a despreading part 806, atotal noise power estimation part 808, a pilot received power estimationpart 810, a multipath interference removing part 812 and an averagingpart 814.

The despreading part 806 despreads signals received by a correspondingreceiving antenna so as to output pilot signals Z_(nml)(s) that aredespread for each transmission antenna and for each path, wherein n is aparameter indicating a transmission antenna, m is a parameter indicatinga receiving antenna (m=1 for the noise power estimation part 802-1), lis a parameter indicating a path, and s is a parameter indicating asymbol number.

The total noise power estimation part 808 calculates total noise powersI_(nml) each being proportional to dispersion of the despread pilotsignal for each path according to the following equation:

$I_{n,m,l} = {\frac{S}{S - 1}\left( {\frac{1}{S}{\sum\limits_{s = 0}^{S - 1}\; {{{Z_{n,m,l}(S)} - Z_{n,m,l}^{\prime}}}^{2}}} \right)}$

wherein Z′_(nml) is an amount calculated by the following equation:

${Z_{n,m,l}^{\prime} = {\frac{1}{S}{\sum\limits_{s = 0}^{S - 1}\; {Z_{n,m,l}(S)}}}},$

wherein this equation means calculating an average value of the despreadpilot signals Z_(nml)(s) for S symbols. In the right side of theequation for calculating the total noise power I_(nml), the term insidethe parentheses indicates an amount of dispersion of the despread pilotsignal Z_(nml). Therefore, the total noise power I_(nml) indicates powerincluding noise arising in the receiver, interference included in thepropagation route and other noise.

The pilot received power estimation part 810 calculates received powersP_(nml) of the pilot signals for each path by the following equation:

$p_{n,m,l} = {{Z_{n,m,l}^{\prime}}^{2} - {\frac{1}{S}I_{n,m,l}}}$

wherein the second term in the right side of the equation indicatesinterference component included in the received power |Z′_(nml)|² of theaveraged pilot signal. Interference component of received power|Z_(nml)|² of the before-averaged pilot signal is evaluated by the abovetotal noise power I_(nml). The interference component included in thereceived power |Z′_(nml)|² that has been averaged for the S symbols isdecreased to 1/S of the interference component of the before-averagedreceived power |Z_(nml)|² due to the averaging calculation. Therefore,1/S is introduced in the second term of the right side of the equation.By using the above equation, the received power P_(nml) of the pilotsignal for each path can be calculated correctly.

The multipath interference removing part 812 removes multipathinterference component from the total noise power I_(nml) so as toestimate the noise power (chip noise power) σ_(nml) ² according to thefollowing equation:

$\sigma_{n,m,l}^{2} = {N_{SF}\left( {I_{n,m,l} - {\frac{1}{N_{SF}}{\sum\limits_{n^{\prime} - 1}^{N}\; {\sum\limits_{\underset{l^{\prime} \neq 1}{l^{\prime} = 1}}^{L}\; {\left( {1 + \alpha} \right)P_{n^{\prime},m,l^{\prime}}}}}}} \right)}$

wherein α indicates a predetermined power ratio between the pilot signaland the data signal (refer to FIG. 6), (1+α)P_(n′ml′) inside theparentheses in the right side indicates a total power (of the pilotsignal and the data signal) of a l′-th path in signals from a n′-thtransmission antenna, summation for l′ is performed for all pathsexcluding the own l-th path, summation for n is performed for alltransmission antennas, N_(SF) indicates a spreading ratio or a chip rateof the pilot signal, and N_(SF) is 256, for example. In the DS-CDMAscheme, since the interference component in a propagated signal isreduced to 1/(spreading ratio), 1/N_(SF) is introduced in the secondterm (indicating multipath interference) inside the parentheses in theright side. In addition, the right side is multiplied by NSF to estimatethe chip noise power.

The averaging part 814 averages the noise powers σnml2 for pluraltransmission antennas (n) and the paths (l). Further, the averaging part804 averages the noise powers for the plural receiving antennas (m) tofinally estimate a desired noise power σ2. Since the effect of themultipath interference is removed in the obtained noise power, the noisepower is estimated more correctly compared with the conventionaltechnology. Therefore, the weight generation part 310 of FIG. 3 cancalculate the weight properly. In addition, from the viewpoint offurther improving the precision, the averaging part 814 or 804 mayupdate the noise power recursively by using an oblivion coefficient a.That is, the noise power can be updated according tok+12=a·σk2+(1−a)σk−12. The updating method of the noise power is notlimited to the above mentioned method using the oblivion coefficient.The update can be performed by using other recurrence formulas. Further,weighting coefficient can be appropriately adjusted when averaging thenoise powers among antennas.

Third Embodiment

FIG. 9 shows a conceptual diagram of a multistage signal detectionapparatus according to an embodiment of the present invention. Thesignal detection apparatus includes serially connected plural blockseach including a two-dimensional MMSE equalizing part 902, a multipathinterference (MPI) replica generation part 904, and a subtracting part906. The two-dimensional MMSE equalizing part 902 includes a channelestimation part 908, a chip noise estimation part 910, a two-dimensionalMMSE weight calculation part 912 and a weight multiplication part 914.These components in the two-dimensional MMSE equalizing part 902correspond to the channel estimation part 302, the noise estimation part304, the weight generation part 310 and MMSE equalizing part 312respectively. Either of noise estimation methods in the first embodimentand the second embodiment can be adopted as a noise estimation methodused in the chip noise estimation part 910.

The MPI replica generation part 904 regenerates multipath componentsbased on the channel estimation result and the transmission signals thathave been separated. For example, signal components (corresponding topath 2 in the example of FIG. 5) of all paths other than a target pathare regenerated. The regenerated signal components are called MPIreplicas. The subtraction part 906 subtracts the MPI replicas from thereceived signals. In the signals in which the MPI replicas have beensubtracted, the ratio of the target path is increased. Therefore, byperforming channel estimation and signal separation based on the signalin which the MPI replicas have been subtracted, estimation accuracy andseparation accuracy can be improved. In the same way, based on theafter-subtracted signal in the previous stage, channel estimation andsignal separation are performed so as to generate MPI replicas, and theMPI replicas are subtracted from the received signals, and the signalsare supplied to the channel estimation part of a next stage.Accordingly, the channel estimation accuracy and the signal separationaccuracy can be largely improved.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the invention.

The present application contains subject matter related to JapanesePatent Application No.2004-144181, filed in the JPO on May 13, 2004, theentire contents of which are incorporated herein by reference.

1. A noise power estimation apparatus, comprising: a part for obtaininga total noise power that is in proportion to dispersion of despreadpilot signals; a part for subtracting an amount that is in proportion tothe total noise power from an average power of the despread pilotsignals so as to obtain a received power of the pilot signal for eachpath; a part for obtaining a multipath interference component based onthe received power of the pilot signal for each path and a predeterminedpower ratio between the pilot signal and a data signal; and a part forsubtracting the multipath interference component from the total noisepower so as to obtain a noise power.
 2. A signal detection apparatusadopting the MMSE scheme, in which received signals that are transmittedby plural transmission antennas and that are received by one or morereceiving antennas are multiplied by receiving weights so as to separatethe received signals to signals for each transmission antenna, whereinthe signal detection apparatus includes an noise estimation apparatus,and the signal detection apparatus calculates the receiving weights byusing a noise power estimated by the noise power estimation apparatus,the noise power estimation apparatus comprising: a part for obtaining atotal noise power that is in proportion to dispersion of despread pilotsignals; a part for subtracting an amount that is in proportion to thetotal noise power from an average power of the despread pilot signals soas to obtain a received power of the pilot signal for each path; a partfor obtaining a multipath interference component based on the receivedpower of the pilot signal for each path and a predetermined power ratiobetween the pilot signal and a data signal; and a part for subtractingthe multipath interference component from the total noise power so as toobtain a noise power.
 3. A noise power estimation method, comprising thesteps of: obtaining a total noise power that is in proportion todispersion of despread pilot signals; subtracting an amount that is inproportion to the total noise power from an average power of thedespread pilot signals so as to obtain a received power of the pilotsignal for each path; obtaining a multipath interference component basedon the received power of the pilot signal for each path and apredetermined power ratio between the pilot signal and a data signal;and subtracting the multipath interference component from the totalnoise power so as to obtain a noise power.